From thekitchn...
//....How Kosher Salt Got Its Name
Any salt can be kosher if it’s produced under kosher supervision, but it’s not because of Jewish dietary guidelines that kosher salt got its name. In fact, something labeled “kosher salt” can actually not be kosher at all!
Kosher salt’s original purpose was really to kosher meat, meaning to remove the blood from meat, so it’s really koshering salt. Certain salt companies labeled the boxes of this coarse salt kosher salt rather than koshering salt, and the name stuck....//